The present invention relates to methods of making patterns in photoresist using photolithography. More particularly, this invention is directed at making groove patterns in photoresist having line width in the submicron range.
The formation of highly precise and specific groove patterns of small dimensions (micrometer or less) in material is widely used in diverse technologies such as optical gratings, semi-conductor chips, surface wave devices etc. Mass production of such printed patterns on substrates can be achieved by photolithography. In this technique, photosensitive material (photoresist) is exposed, via a reticle or xe2x80x9cmaskxe2x80x9d exhibiting the desired pattern, to electromagnetic radiation followed by chemical development to remove the photoresist.
The established photolithographic equipment to print patterns with sub-micron line width is the so-called stepper. Typically a so-called 5xc3x97 stepper in which the pattern on the reticle is produced at five times the size of the final pattern can be used to provide high resolution. The image of the pattern is then projected through lenses to produce the pattern on the photoresist at the appropriate scale.
Most devices that require grooves with small line widths also exhibit high line density with spacing between the grooves approaching that of the electromagnetic radiation source""s wavelength. As a result, the radiant energy cannot pass through a slit that is smaller than the wavelength. Consequently, it has not been practical to mass produce devices with line widths of about 0.25 xcexcm or less using a 5xc3x97 stepper.
The diffraction problem can also be addressed by providing special filters to intercept the diffraction before it reaches the photoresist surface (see for example U.S. Pat. No. 4,947,413). However, adjustment of filters may be difficult and time consuming.
The diffraction problem has been partly overcome for the making of surface wave devices as described in U.S. Pat. No. 5,972,568 wherein a method is provided that increases the spacing between slits on the reticle by separating the pattern in two or more subsets. The final pattern is then obtained by exposing the photoresist to each subset. However, the method of the ""568 patent is restricted to surface wave devices because their very specific surface patterns, which include connection rails, prevent the application of the method to other devices exhibiting different patterns.
The resolution of the photolithography technique is also limited by the width of the slits forming the pattern on the reticle. The width of the slits necessary to achieve the desired pattern is, in turn, limited by the wavelength of the electromagnetic radiation used. For instance, most photolithographic apparatus for producing sub-micron patterns use deep ultraviolet (DUV) radiation. The wavelength of this radiation is the range of 180-250 nm and enables the use of narrow slits. However, as the wavelength of the source approaches the width of the slits, the radiation cannot penetrate the slit.
In general, the printing of small linewidth patterns (less than 0.25 xcexcm) is achieved by using direct writing with electron beams. However, this method is very slow and, consequently, expensive.
It is thus desirable to develop a technique that would further reduce the diffraction caused by the high line density and narrow slits of the reticle.
According to one aspect of the present invention, there is provided a method of making parallel grooves in a photoresist coating on a substrate. The method comprises the steps of: a) providing a reticle with a pattern of parallel lines each line being opaque to electromagnetic radiation, the distance between the adjacent two lines being greater than the width of the line; b) subjecting said photoresist to a first exposure of electromagnetic radiation of a selected wavelength via said reticle with a dose less than is required to fully sensitize said photoresist, wherein, by means of the width of said line and the distance between the adjacent two lines, an alternate pattern of an unexposed area and a partially exposed area is formed on the photoresisit coating; c) shifting said reticle relative to said photoresist coating or vice versa such that each line of said reticle is projected in each said partially exposed area; d) subjecting said photoresist to a second exposure of electromagnetic radiation of a selected wavelength via said shifted reticle with a dose less than is required to fully sensitize said photoresist to development such that the total dose of the two exposures is equal to or greater than the dose required to fully sensitize the photoresist, wherein that area of each said partially exposed area that is not projected by the line of said shifted reticle is fully exposed; and e) developing the photoresist, wherein said fully exposed area is removed from the photoresist whereby a parallel grooves can be formed in a photoresist coating.